Electron Configuration & Orbital Diagram

Electron configuration of any element or ion: orbital boxes, unpaired electrons, exceptions.

How it works

Type an element or an ion — Fe, Fe3+, Cu+, O2-, 26 or “iron” — and get its ground-state electron configuration in full and in noble-gas shorthand, the orbital box diagram drawn with Hund’s rule, the number of unpaired electrons (paramagnetic or diamagnetic), the electrons per shell and the quantum numbers of the last electron. The 20 elements whose measured ground state breaks the Aufbau rule, such as chromium, copper, palladium and gold, use the NIST configuration and say what the rule would have predicted. Part of the Proofline math notebook.

Subshells fill in Madelung order (increasing n + l, then n): 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p. For Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, La, Ce, Gd, Pt, Au, Ac, Th, Pa, U, Np, Cm and Lr the measured ground state of the NIST Atomic Spectra Database replaces the prediction.
Cations lose electrons from the subshell with the highest n first (for equal n the highest l), so transition metals lose their s electrons before d electrons (Fe²⁺ = [Ar] 3d⁶). Anions add electrons in Madelung order.
Orbital boxes follow Hund’s rule: within a subshell each orbital (mℓ = −l … +l, left to right) receives one spin-up electron before any is paired. Unpaired electrons = Σ min(e, 2(2l + 1) − e) over the subshells.
Verified against the PubChem/NIST configurations of all 118 elements (the only difference is lawrencium, where NIST gives 7p¹ and PubChem still 6d¹).

Configurations are for free atoms and ions in the gas phase; in compounds ligands can pair electrons (low-spin complexes). Configurations beyond meitnerium are theoretical predictions.

Sources

A. Kramida, Yu. Ralchenko, J. Reader and NIST ASD Team: NIST Atomic Spectra Database — ground levels and ionization energies of the neutral atoms (National Institute of Standards and Technology)
P. Atkins, J. de Paula, J. Keeler: Atkins’ Physical Chemistry, 11th ed. (Oxford University Press, 2018) — the building-up principle, Hund’s rule; real gases and the van der Waals equation (Topic 1C) (Oxford University Press)
S. Kim et al.: PubChem Periodic Table and Element pages — machine-readable element data compiled from NIST, LANL, Jefferson Lab, IUPAC and IAEA, Chemistry Teacher International 3 (2021) 57–65 (National Center for Biotechnology Information (NCBI), U.S. National Library of Medicine)

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Write the electron configuration of any element or ion

Get the ground-state electron configuration of an atom or ion with orbital boxes, unpaired electrons and the quantum numbers of the last electron, including the exceptions.

What it is for

Writing electron configurations is one of the first skills in chemistry, and one of the easiest to get wrong. The filling order (the Aufbau or Madelung rule) works for most atoms, but there are twenty exceptions, such as chromium and copper. Ions of transition metals are another trap, because they lose their outer s electrons first.

Type an element symbol, a name or an atomic number, optionally with a charge: Fe, iron, 26, Fe3+ or O2-. The tool returns the noble-gas shorthand and the full configuration, the electrons per shell, the number of unpaired electrons and whether the species is paramagnetic. It also shows the orbital boxes drawn by Hund's rule and the four quantum numbers of the last electron.

Exceptions use the measured ground states from the NIST Atomic Spectra Database, and the tool tells you what the filling rule would have predicted. Configurations are for free atoms in the gas phase. In compounds and crystals the picture can differ.

Worked example

Enter Fe. The result is [Ar] 3d⁶ 4s² with 4 unpaired electrons, so iron is paramagnetic. The shells hold 2, 8, 14 and 2 electrons. The orbital boxes show five 3d boxes: one pair and four single arrows.

Now enter Fe3+. The configuration becomes [Ar] 3d⁵ with 5 unpaired electrons. A note explains why: transition-metal ions lose their 4s electrons before their 3d electrons, so the answer is not [Ar] 3d³ 4s². Mn2+ gives the same [Ar] 3d⁵.

Enter Cr. The filling rule predicts [Ar] 3d⁴ 4s², but the measured ground state is [Ar] 3d⁵ 4s¹ with 6 unpaired electrons. Copper is similar: [Ar] 3d¹⁰ 4s¹ instead of 3d⁹ 4s². For gadolinium you get [Xe] 4f⁷ 5d¹ 6s² with 8 unpaired electrons.

How to read the result

Configuration is the shorthand with the nearest noble-gas core. Full configuration spells out every subshell.
Unpaired electrons are counted from the orbital boxes. If any are unpaired the species is paramagnetic, otherwise diamagnetic.
Electrons in the outer shell counts the electrons with the highest n, which are the valence electrons for main-group elements.
The subshell table gives the electrons, capacity, unpaired count and position in the filling order of each subshell.
Quantum numbers of the last electron follow the usual school convention: boxes are filled from mℓ = −l, spin up first. Other books use the opposite convention.

Tips and common mistakes

Write charges after the symbol: Fe3+, Fe^3+ and Fe+3 all work. O2- is the oxide ion.
Order the shorthand by shell (3d before 4s), as here and in NIST tables, or by filling order. Both are accepted in most courses.
An ion cannot lose more electrons than the atom has. H2+ is rejected.
Heavy elements beyond Z ≈ 104 have predicted configurations only.
Use the full orbital diagram for small atoms and the valence diagram for heavy ones; it shows only the subshells outside the core.

FAQ

Why is chromium [Ar] 3d⁵ 4s¹?

The 3d and 4s energies are very close in chromium. A half-filled d subshell lowers the electron repulsion and exchange energy enough to make 3d⁵ 4s¹ the ground state.

Which elements are exceptions?

Twenty: Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, La, Ce, Gd, Pt, Au, Ac, Th, Pa, U, Np, Cm and Lr. The tool marks each one and shows the filling-rule prediction.

How are anions handled?

Extra electrons go into the next free places in the filling order, so N3- and O2- both reach the neon configuration.

Is the magnetism a prediction for the solid metal?

No. It describes the free atom or ion. Bulk magnetism such as ferromagnetism in iron depends on the crystal.

Guide last updated: November 8, 2026

For scripts and AI agents

JSON API: POST https://prooflineelectronconfig.usefulweb.app/api/v1/calculate (same calculation as this page)
Interface description: OpenAPI 3.1 · AI guide: llms.txt
MCP server (Streamable HTTP): https://prooflineelectronconfig.usefulweb.app/mcp · tool name proofline_electron_config

Example request

curl -s -X POST https://prooflineelectronconfig.usefulweb.app/api/v1/calculate \
  -H 'content-type: application/json' \
  -d '{"species":"Fe"}'

Feedback · ideas & bug reports welcome